REFRACTORY PHASE IN CYCLIC-AMP-RESPONSIVE TRANSCRIPTION REQUIRES DOWN-REGULATION OF PROTEIN KINASE-A

Cyclic AMP (cAMP) stimulates the expression of numerous genes through the protein kinase A (PK-A)-mediated phosphorylation of the nuclear fa ctor CREB at Ser-133 (G, A, Gonzalez and M, R Montminy, Cell 59:675-68 0, 1989), Like other signal transduction pathways, cAMP induces gene e xpression with burst-attenuation kinetics; cAMP-dependent transcriptio n and CREB phosphorylation peak within 30 min and decline steadily ove r the next 4 to 6 h via the protein phosphatase 1-mediated dephosphory lation of CREB (M. Hagiwara, A, Alberts, P, Brindle, J, Meinkoth, J, F eramisco, T, Deng, hi, Karin, S, Shenolikar, and M. Montminy, Cell 70: 105-113, 1992), Here we characterize a third phase in cAMP-responsive transcription-a refractory period during which hormone-treated cells b ecome transcriptionally unresponsive to subsequent stimulation by cAMP . This refractory period begins 6 to 8 h after stimulation and lasts 3 to 5 days after the removal of hormone, In contrast to the earlier at tenuation phase, transcription of cAMP-responsive genes during the ref ractory period is not restored by inhibitors of protein phosphatase 1. activity, Rather, the establishment and maintenance of this phase rel y on a marked reduction in PK-A catalytic subunit expression at the tr anslational level, As overexpression of C-subunit protein can reactiva te transcription of cAMP-responsive genes during the refractory period , our results suggest that hormone-responsive cells may stimulate, att enuate, and then silence signal-dependent genes through distinct regul atory mechanisms.